A photo relay using a metal-oxide-semiconductor (MOS) transistor is well known. The MOS transistor is driven by a photovoltaic output from a photocoupler circuit as a relay contact. Such a photo relay is disclosed in Japanese Patent Application Publication No. 2002-50952.
The photo relay includes a photo diode coupler circuit, an output MOS transistor, a control resistor, a control transistor, a discharge resistor, and a discharge transistor. An impedance between a drain and a source of the output MOS transistor varies when a capacitance between a gate of the output MOS transistor and the source is charged by a photovoltaic output. The control resistor and the control transistor control charging and discharging of gate charge of the output MOS transistor. The gate charge discharged from the output MOS transistor flows in the discharge resistor and the discharge transistor.
The control resistor generates a bias voltage for turning on or off the control transistor depending on the photovoltaic output from the photocoupler circuit. The discharge resistor generates a bias voltage for turning on or off the discharge transistor depending on the photovoltaic output from the photocoupler circuit.
In the photo relay, the control resistor and the discharge resistor need to have high resistances in the order of MΩ to generate a necessary bias voltage because a current generated by a light receiving element is in the order of μA.
The control resistor and the discharge resistor with such high resistances is formed in a semiconductor chip. The control resistor and the discharge resistor occupy a large area in the semiconductor chip. As a result, there is a problem that integration of the semiconductor chip may be difficult, as the number of such resistors is large.
In addition, for the high-speed switching of the output MOS transistor, it is important to control the resistances of the control resistor and the discharge resistor. However, it may be difficult to form a plurality of resistors of high resistances in a semiconductor chip with high precision.